Check the fuses - pull them out and test. The TP4 is AC, since you have GI, I'm guessing you were trying to read DC volts instead of AC volts.

You also need to replace those connectors to the board. They have been hacked, and its likely you have a bad connection going to the rest of the machine.
Concentrate on the rectifier board first.

The Harbor Freight meter does not measure ESR.(Equivalent Series Resistance) You need a special meter for that. Its cheaper to replace C23 than buying a meter.

Have you replaced the connectors yet?

Bob-

I would definitely repin J3, and since there is battery damage, take a close look at the pins at MPU J4. I have had several instances where the corrosion crawled into the pins on J4.

I would also reflow the connections on the ground plane for those components - they all look like cold solder joints, and clean the board with some vinegar - it looks like the corrosion has not stopped.

This may not fix your immediate problem, (Repinning J3 might) but it will help make the whole platform more solid.